Fine Grained Deep Ocean Survey Techniques

ABSTRACT A deeply towed system for fine scale deep ocean surveys has been assembled by the Marine Physical Laboratory over the last 15 years. It consists of precision echo sounder, sub-bottom penetration sounder, side-looking sonar, cameras, TV, magnetometer and a variety of other specialized measurement and sampling devices all operated on a coaxial electrical/mechanical towing cable. Local navigation to 5 m precision is provided by an acoustic transponder system. Recent advances have included digital side-looking sonar, dual frequency sub-bottom penetration system, an advanced transponder/echo repeater and a delay device for maintaining navigation when local topography or refraction blank out sound paths to transponders. The system makes it possible to map manganese nodule distribution variations to locate small features that would impede sea floor mining operations, and to provide after-the-fact assessment of deep sea floor mining effects and effectiveness. INTRODUCTION Studies of the fine scale nature of the sea floor in intermediate depths and deep ocean environments are becoming increasingly important. As it becomes feasible to give serious consideration to exploration of mineral resources in these regimes it becomes essential that we know about their small scale properties. In deep offshore situations these properties not only control our decisions as to how to choose oil drilling locations, they also control the routing of sea floor pipelines which can link distant offshore drill sites to shore side storage terminals. In the deep ocean, manganese deposits pose similar questions, including ability to assay the extent and uniformity of nodule beds. Such things as the size and frequency of occurrence of outcropping rock and other small-scale irregularities in the terrain can have a significant effect on choices in the design and operation of actual mining machinery. The survey techniques which are used to determine these properties can also play a role in evaluating effectiveness of sea floor mining operations and in the maintenance and repair of sea floor equipment, particularly by making fine scale surveys following completion of mining operations. Another aspect of the need for such fine scale survey technology arises from need for regulatory bodies to understand the sea floor environment in order to be able to make intelligent decisions as to how to discharge their responsibilities in relation to the users and in negotiations on the international scene. Interestingly enough, while all of the aspects above are quite plausible and might be thought important, they are not the considerations that have actually supported the development and application of deep ocean fine scale survey technology. Instead it has been the combination of the Navy's interest in deep sea search and salvage, and in the acoustic properties of the sea floor, as well as the basic research activities of the National Science Foundation in marine geology and geophysics that have brought our present capabilities - both technological and operational into being, and continue to provide the primary impetus for further development.